In liquid crystal displays (LCDs) it is often required to control the alignment of the LC medium. This is typically achieved using an alignment layer. Reviews of conventional alignment techniques are given for example by 1. Sage in “Thermotropic Liquid Crystals—Applications and Uses Vol. 3” edited by B. Bahadur, World Scientific Publishing, Singapore 1992, pages 1-63. A review of alignment materials and techniques is given by J. Cognard, Mol. Cryst. Liq. Cryst. 78, Supplement 1 (1981), pages 1-77. These alignment techniques and materials can be used to align both conventional LC materials and polymerizable LC materials, also known as reactive mesogens (RM) and reactive mesogen mixtures (RMMs).
Typically, high surface energy substrates are used to obtain homogeneous (planar) alignment of LCs. For example, rubbed polyimide, TAC or PET films are often used to obtain planar alignment of LCs. Other, less common polar materials, e.g. cellulose nitrate have also been reported as alignment layers, for example in U.S. Pat. No. 5,805,253.
In prior art various alignment methods have been used to achieve homeotropic (perpendicular) alignment of LC materials. The simplest is to use a substrate with a low surface energy (e.g. PTFE). In this case the energy of the system is minimised by having the LC molecules in contact with each other rather than with the surface, resulting in homeotropic alignment. Unfortunately, this approach is limited and there are few examples of low surface energy, plastic substrates of the optical quality required for use in LCDs.
In prior art there are examples of other ways to homeotropically align thin layers of LC materials, but these usually require an alignment layer. For example, a substrate is coated with a surfactant that ‘seeds’ homeotropic alignment (e.g. lecithin). Other examples exist where a plastic substrate is coated with a low surface energy alignment layer, as disclosed for example in U.S. Pat. No. 5,456,867, U.S. Pat. No. 6,379,758 and U.S. Pat. No. 6,816,218. It has also been suggested in prior art to use a thin, polymerised layer of homeotropically aligned RMM as an alignment layer for inducing homeotropic alignment of LCs, as disclosed for example in WO 02/44801. However, in the method according to WO02/44801 the initial RMM still required the use of an alignment layer (silica treated PET) to achieve the initial homeotropic alignment.
Unfortunately, the use of an alignment layer adds extra cost to the production of homeotropic films. This problem was investigated and partially overcome in US 2004/0263736 where a homeotropic LC layer is aligned directly on top of a planar RMM layer. This combination of RMM films offers one solution to compensate LCDs of the in-plane-switching (IPS) mode. However, it is most desirable to be able to homeotropically align an RMM on any substrate.
One aim of the present invention is to provide an alignment layer, and methods and materials for its preparation, which provides uniform and stable alignment of LCs applied thereon, is easy to manufacture, in particularly for mass production, and does not have the drawbacks of prior art alignment layers and materials described above. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The inventors have found that these aims can be achieved by using materials and methods as described in the present invention. Thus, the present invention provides an RM material that aligns homeotropically on both low and high surface energy substrates, where the substrate can be inorganic, or organic (e.g. plastic film, polymerized RM or LC film, etc.).